1. Field of the Invention
The present invention relates to a process for purifying ammonia. More particularly, the present invention pertains to a process for purifying ammonia, capable of removing impurities such as oxygen, carbon dioxide and water to an extremely low concentration that are contained in ammonia, for instance, crude ammonia available on the market for industrial use or crude ammonia which is recovered from a gallium nitride compound semiconductor process.
2. Description of the Related Arts
Ammonia is used together with silane for the formation of a silicon nitride film and also together with triethyl gallium and the like for the formation of a gallium nitride film each in a semiconductor process. In particular, a production process for a gallium nitride compound semi-conductor which is widely employed as an element for a light emitting diode and laser diode is put into practice usually by subjecting a gallium nitride compound to vapor phase growth on a substrate made of sapphire or the like by means of MOCVD process. Examples of feed gases that are used in the above-mentioned process include a trimethylated group III element such as trimethyl gallium, trimethyl indium and trimethyl aluminum and besides ammonia, that is, a compound of a group V element. Accompanying the progress in film formation technique in recent year, ammonia to be used therein is forcefully required to have an extremely high purity. Because of the aforesaid requirement and a large amount to be used, there is required to develop a process for purifying ammonia capable of continuously supplying highly pure ammonia.
In general, ammonia available on the market for industrial use contains oxygen, carbon dioxide, moisture, etc. Ammonia having a relatively high purity is marketed in the form obtained by further distilling or rectifying the ammonia available on the market, or in the form obtained by further diluting with a highly pure inert gas. However, since ammonia to be used as a feed gas in a semiconductor process and the like as mentioned above is required to have an extremely high purity, there has hitherto been developed a process for further purifying ammonia having a relatively high purity obtained by further distilling or rectifying ammonia for industrial use.
As a process for purifying ammonia, there have previously been available {circle around (1)} an ammonia purifying process in which carbon dioxide gas in crude ammonia is adsorptively removed by passing crude ammonia through a solid alkali layer maintained at such a temperature as higher than the temperature at which the solid alkali is no longer dissolved due to the deliquescent property of the solid alkali and also at such a temperature as lower than the melting point thereof {refer to Japanese Patent Application Laid-Open No. 24737/1994 (Heisei 6)} and {circle around (2)} an ammonia purifying process in which moisture in crude ammonia is removed by bringing the crude ammonia into contact with BaO as a simple substance or a mixture comprising BaO as s principal component under the condition of substantially room temperature {refer to Japanese Patent Application Laid-Open No. 142833/1997(Heisei 9)}.
In addition, there have also been developed {circle around (3)} an ammonia purifying process in which oxygen contained in crude ammonia is removed by bringing the crude ammonia into contact with a catalyst comprising nickel as a principal component {refer to Japanese Patent Application Laid-Open No. 124813/1993(Heisei 5)}, {circle around (4)} an ammonia purifying process in which carbon monoxide and carbon dioxide that are contained in crude ammonia are removed by bringing the crude ammonia into contact with a catalyst comprising nickel as a principal component {refer to Japanese Patent Application Laid-Open No. 107412/1994 (Heisei 6)}, etc.
However, the ammonia purifying processes {circle around (1)} and {circle around (2)} have suffered from the disadvantages in that although the process {circle around (1)} can remove only carbon dioxide gas and the process {circle around (2)} can remove only moisture, each of the processes must be frequently combined with an other process to purify the ammonia in order that the process is employed in a semiconductor process. Moreover with regard to the ammonia purifying processes {circle around (3)} and {circle around (4)}, in the case of a high contact temperature between the catalyst and ammonia, there is a fear of hydrogen generation due to ammonia decomposition, hence necessitating to purify the ammonia by maintaining the contact temperature at around ordinary temperature. Further, in the above-mentioned gallium nitride compound semiconductor process, because of low decomposition efficiency of ammonia, an extremely large amount thereof is required to be used as the feed gas compared with trimethylated group III element such as trimethyl gallium, trimethyl indium and trimethyl aluminum. The ammonia employed as the feed gas in the foregoing semiconductor process is highly pure ammonia which is obtained by distilling or rectifying ammonia for industrial use, or is ammonia obtained by further purifying the aforesaid highly pure ammonia. Thus, it has been impossible to employ ammonia available on the market for industrial use as it is.
In addition thereto, the ammonia available on the market subjected to distillation or rectification costs several ten times as compared with ammonia available on the market for industrial use.
By the reasons mentioned hereinbefore, the ammonia available on the market subjected to distillation or rectification suffers from the drawback in that it leads to an extremely high running cost, particularly in a gallium nitride compound semiconductor process, and what is more, most of it has been discarded in a large amount without being reacted or utilized in the above-mentioned process. In such circumstances, it has eagerly been desired to develop an ammonia purification process capable of continuously supplying the semiconductor process with ammonia, and at the same time, of contributing to the production of the semiconductor at a reasonable cost.
Further, most of the ammonia having extremely high purity, after being exhausted from the semiconductor process, has been treated by a wet absorption process, a combustion treatment process, a dry adsorption process or a decomposition treatment process. However, the above-mentioned processes involves such problems still remain unsolved as described hereunder. The wet absorption process which comprises neutralizing the ammonia by passing it through an acidic aqueous solution is problematic in the post treatment of by-produced ammonium salts. The combustion treatment process which comprises burning the ammonia by introducing into the flame of a fuel such as propane is problematic in fuel consumption, NOx treatment and CO2 generation. The dry adsorption process which comprises bringing the ammonia into contact with a chemical agent having chemical reactivity with the ammonia is problematic in high running cost due to the use of an expensive chemical agent. The decomposition treatment which comprises decomposing the ammonia into nitrogen and hydrogen by bringing the ammonia into contact with an ammonia decomposition catalyst is problematic in high running cost due to large electric power consumption.
An efficient recovery and recycle of the ammonia thus used, if made possible, can contribute to not only the effective utilization of a resource but also environmental preservation.
Under such circumstances, a general object of the present invention is to provide a process for purifying ammonia, capable of removing impurities such as oxygen, carbon dioxide and moisture that are contained in ammonia each in a slight amount to an extremely low concentration, and of preventing hydrogen from being generated by the decomposition of ammonia even at a relatively high contact temperature.
Another object of the present invention is to provide a process for purifying ammonia, capable of readily producing ammonia which is usable as a feed gas for a gallium nitride-compound semiconductor from crude ammonia available on the market for industrial use or crude ammonia which is recovered from a gallium nitride compound semiconductor process.
Other objects of the present invention will be obvious from the text of this specification hereinafter disclosed. Under such circumstances, intensive extensive research and investigation were made by the present inventors in order to achieve the above-mentioned object. As a result, it has been found that oxygen, carbon dioxide and moisture contained in crude ammonia as impurities can be removed to as low as 0.1 ppm or to less than 0.01 ppm by bringing the crude ammonia into contact with a catalyst comprising manganese oxide as an effective ingredient, preferably further with a synthetic zeolite having a pore diameter in the range of 4 to 10 xc3x85 or equivalent and that manganese oxide is a catalyst which is less prone to decompose ammonia. Such finding led to the ammonia purification process according to one aspect of the present invention.
In addition, it has been found that, if it is made possible to remove oxygen, carbon dioxide and moisture which exert evil influence upon a gallium nitride compound semiconductor process each to an extremely low concentration from crude ammonia available on the market for industrial use or from crude ammonia recovered out of a gallium nitride compound semiconductor process, then the purified ammonia can be used as a feed gas to a gallium nitride compound semiconductor.
Moreover, it has been found that oxygen, carbon dioxide and moisture contained as impurities in crude ammonia available on the market for industrial use or in crude ammonia recovered from a gallium nitride compound semiconductor process can be removed to as low as 0.1 ppm or to less than 0.01 ppm by bringing the crude ammonia into contact with a catalyst comprising manganese oxide and/or nickel as an effective ingredient, and further with a synthetic zeolite having a pore diameter in the range of 4 to 10 xc3x85 or equivalent. Such finding led to the ammonia purification process according to another aspect of the present invention.
That is to say, the present invention provides:
1. A process for purifying ammonia which comprises bringing crude ammonia into contact with a catalyst comprising manganese oxide as an effective ingredient to remove oxygen and/or carbon dioxide that are contained as impurities in the foregoing crude ammonia.
2. A process for purifying ammonia which comprises bringing crude ammonia into contact with a catalyst comprising manganese oxide as an effective ingredient, and thereafter with a synthetic zeolite having a pore diameter in the range of 4 to 10 xc3x85 to remove at least one impurity selected from the group consisting of oxygen, carbon dioxide and moisture that are contained in the foregoing crude ammonia.
3. A process for purifying ammonia to be used as a feed gas for a gallium nitride compound semiconductor which comprises bringing crude ammonia available on the market for industrial use into contact with a catalyst comprising manganese oxide as an effective ingredient, and thereafter with a synthetic zeolite having a pore diameter in the range of 4 to 10 xc3x85 to remove at least one impurity selected from the group consisting of oxygen, carbon dioxide and moisture that are contained in the foregoing crude ammonia.
4. A process for purifying ammonia to be used as a feed gas for a gallium nitride compound semiconductor which comprises bringing crude ammonia available on the market for industrial use into contact with a catalyst comprising nickel as an effective ingredient, and thereafter with a synthetic zeolite having a pore diameter in the range of 4 to 10 xc3x85 to remove at least one impurity selected from the group consisting of oxygen, carbon dioxide and moisture that are contained in the foregoing crude ammonia.
5. A process for purifying ammonia to be used as a feed gas for a gallium nitride compound semiconductor which comprises bringing crude ammonia available on the market for industrial use into contact with a catalyst comprising manganese oxide as an effective ingredient and with a catalyst comprising nickel as an effective ingredient consecutively in this order, and thereafter bringing the ammonia into contact with a synthetic zeolite having a pore diameter in the range of 4 to 10 xc3x85 to remove at least one impurity selected from the group consisting of oxygen, carbon dioxide and moisture that are contained in the foregoing crude ammonia.
6. A process for purifying ammonia to be used as a feed gas for a gallium nitride compound semiconductor which comprises bringing crude ammonia available on the market for industrial use into contact with a catalyst comprising nickel as an effective ingredient and with a catalyst comprising manganese oxide as an effective ingredient consecutively in this order, and thereafter bringing the ammonia into contact with a synthetic zeolite having a pore diameter in the range of 4 to 10 xc3x85 to remove at least one impurity selected from the group consisting of oxygen, carbon dioxide and moisture that are contained in the foregoing crude ammonia.
7. A process for purifying ammonia to be used as a feed gas for a gallium nitride compound semiconductor which comprises the step of bringing crude ammonia available on the market for industrial use into contact with a catalyst comprising manganese oxide as an effective ingredient, and thereafter with a synthetic zeolite having a pore diameter in the range of 4 to 10 xc3x85 and the step of bringing the ammonia from the synthetic zeolite into contact with a catalyst comprising nickel as an effective ingredient, and thereafter with a synthetic zeolite having a pore diameter in the range of 4 to 10 xc3x85 consecutively in this order to remove at least one impurity selected from the group consisting of oxygen, carbon dioxide and moisture that are contained in the foregoing crude ammonia.
8. A process for purifying ammonia to be used as a feed gas for a gallium nitride compound semiconductor which comprises the step of bringing crude ammonia available on the market for industrial use into contact with a catalyst comprising nickel as an effective ingredient, and thereafter with a synthetic zeolite having a pore diameter in the range of 4 to 10 xc3x85 and the step of bringing the ammonia from the synthetic zeolite into contact with a catalyst comprising manganese oxide as an effective ingredient, and thereafter with a synthetic zeolite having a pore diameter in the range of 4 to 10 xc3x85 consecutively in this order to remove at least one impurity selected from the group consisting of oxygen, carbon dioxide and moisture that are contained in the foregoing crude ammonia.
9. A process for purifying ammonia to be used as a feed gas for a gallium nitride compound semiconductor which comprises bringing crude ammonia recovered from a a gallium nitride compound semiconductor process into contact with a catalyst comprising manganese oxide as an effective ingredient, and thereafter with a synthetic zeolite having a pore diameter in the range of 4 to 10 xc3x85 to remove at least one impurity selected from the group consisting of oxygen, carbon dioxide and moisture that are contained in the foregoing crude ammonia.
10. A process for purifying ammonia to be used as a feed gas for a gallium nitride compound semiconductor which comprises bringing crude ammonia recovered from a gallium nitride compound semiconductor process into contact with a catalyst comprising nickel as an effective ingredient, and thereafter with a synthetic zeolite having a pore diameter in the range of 4 to 10 xc3x85 to remove at least one impurity selected from the group consisting of oxygen, carbon dioxide and moisture that are contained in the foregoing crude ammonia.